This project is aimed at investigating the development of the factors that control fetal and neonatal cardiovascular functions. Particular emphasis is placed on the following: a) In vivo investigation of the role of the sympathetic and parasympathetic nervous system as well as of the blood respiratory gases and pH in controlling heart rate, systemic and pulmonary vascular pressures, resistances and flows in the fetal lamb throughout gestation and in the neonatal periods. b) In vitro studies of the reactivity of blood vessels as well as of production, uptake and metabolism of the various neuro- transmitters in the fetal cardiovascular systems at different stages of development. c) Studies of the behavior of the contractile proteins and of the sarcoplasmic reticulum including calcium uptake in the heart of the fetal lamb throughout gestation and after birth. Particular attention is given in these studies to the factors that lead to the right ventricular dominance during intrauterine life and the factors that lead to the left ventricular dominance in the neonatal and adult period of life.